smoke detection Flashcards
Benefit of detecting smoke
it’s an early indication of fire
Where are smoke detectors placed
Cargo holds
toilets
Avionics bay
crew rest area
TRaingle - fire causing agents
- source of ignition
- fuel
- oxygen
Take any of the three away, and fire will go out
Categorization
Level of smoke and fire detection depends on ‘accessibility of the compartments
- How easy the are to access and fix the problem
2 Types of ‘smoke’ detection systems
Optical
Ionizing
Optical detection
Detector is comprised of a light source and sensor arranged in this manner:
In normal conditions
* The sensor can’t detect light source
In smoke condition (particles are present)
* light reflects off the particles and is detected by the sensor
* ** Two or more detectors both sensing** the smoke simultaneously will trigger an alarm
* a labyrinth optical detector - light reflects towards the photsensitive cell and the labyrinth has a shield
How to test if smoke detector is working
In cargo smoke panel, you can test by doing this:
- once you press the test button
- test lamp shines at the sensor
- this triggers the signal and illuminates the caption
Ionizing
A system with ionizing chambers
contains radio active material that emit **alpha particles - they ionize the air **in the chambers.
In normal conditions:
* a potential difference in ionized air allows a small electrical current to flow between the plates of the dector
In smoke conditions
* smoke particles enter the chamber.
* They get absorbed by alpha particles
* ionization level decreases, consequently the current flow decreaes
* Once current flow drops to a certain leve, a warning is triggered.
Fire detection and extinguishing system in
2 key areas
cargo compartment
engine bay
main wheel wells
Auxillary power unit
3 vulnerable zones in an engine bay
IN A 3 ZONE SYSTEM
Requires both fire detection and extinguisher
Zone 1 - Most sensitive to fire
Area of hydraulic pumps, fuel pumps, fuel control unit
Zone 2
compressor & fan area
uncontained blade area can cause fire
Generates more heat than zone 1
Zone 3
combustion and turbine sections
Each zone is spearated by fire proof Bulkhead/FIREWALLS - stainless steel or titanium
IN A 2 ZONE SYSTEM
Zone 1 & 2 = Cool zone
Zone 3 = Hot zone
Auxillary Power Unit
Also an engine
- APU bay - located at the back of the aircraft
- Has it’s own fire detection and protection system
- Automatic
- APU shuts down in case of fire
- Zone 1
Engine Fire detection systems
Continuous Loop
Gas filled detector
Bimetallic Overheat sensors
Continous Loop Engine Fire Detection System (FIRE WIRE)
Operates on the principle of:
* Wire to detect resistance and capacitance when heated
* Therefore there is a
Composed of:
1. Controller monitor
2. Detection loops (stainless steel, salt,nickle core)
* negative coefficient of resistance loop
* positiove coefficient of capacitance loop
Location:
* engine nacelles
* loops are wound around engine components on the engine casing
**Each engine is equipped with: **
* A pair of fire detection loops
* A pair of overheat detection loops
Set up:
2 loops are used: Loop A & Loop B
independent of each other
have their own detection controller
How it works - in resistance system
* current flows in a continuous loop
* If temp rises, the salt resistance reduces
* flow of current between electrode and out tube increases
* At a predetermined temperature, sufficient current will flow and warning system will operate
* If temp drops to a certail value, the system will reset and alarm stops
* This works when there is an isolated temperature increase due to hot gas leak or fire
How it works - in capacitance system
* current flows in a continuous loop
* If temp rises, the capacitance increase
* feedback current increases
* when it increases at a predetermined level, warning system will operate
* If temp drops to a preset value, the system will reset and alarm stops
* This works when there is an isolated temperature increase due to hot gas leak or fire
Detecting faults:
* short circuit between the case and inner elctrode may give a false fire warning
* When a disrepency cannot detect heat
* Ex: If there is a rapid decrease in resistance and corresponding increase in capacitance
Fault Alert
* Amber FAULT caption is illuminated for the relevant loop A or Loop B push button
* Amber Loop light illuminates
What to do after seeing the alert
* Selecting the faulty loop off restores the fire detection capability.
* You must isolate the defective loop because a malfunctioning loop that isn’t turned off, will prevent proper fire alerting
Continous Loop Engine Fire Detection FAULT
Scenario 1:
(low resitance) and (low capacity)
(fire) and (no fire) = fault
Scenarion 2:
Sensing element is crushed, inner core doesn’t touch the other casing
Continous Loop Engine Fire Detection FAULT/Alarm/no effect
Scenario 1: FAULT
(low resitance) and (low capacity)
(fire) and (no fire)
Scenario 2: No Reaction
Sensing element is crushed, inner core doesn’t touch the other casing
Scenarion 3: fire alarm is triggered
Inner core contacts the outer casing
Scenario 4: fire alarm is triggered
Sensing element is crushed,casing is touched
Bimetallic overheat sensors
Located in:
* Used to protect curcial spots on the airframe
Operates on the principle of:
* different metals have different coefficient of expansion
Composed of:
* Outer sensor containing 2 metallic strips with low coefficient of expansion
* metallic strips have an electrical contact each
How it works
* In normal conditions: strips are lightly compressed
* When heated: outer sensor expands, inner strips stretch, contact points touch
* A circuit is created –>signal to flight deck
Notes
May cause slight delay before signal is given
this is to prevent false warning due to vibration
Gas filled detectors
Comprised of:
* Responder Unit with
* 2 pressure switches
* 2 loops (titanium hydride and helium)
How it works
In normal conditions
* gas pressure is 7psi
* gas is connected to the switches in the respondor unit
* Gas operates on averaging gas pressure switch that holds the system integreity switch closed and** alarm switch is open**
If loop breaks - system fault
* helium gas will be released from the core
* Drop in pressure
* opens the integrity switch ->signals fault
If there is spread heat over an area
* the temperature increase is less than the one created by a flame
* But it will be felt by a larger area
* so hydrogen is not going to be released.
* but pressure increase in helium gas
* Closes the alarm switch ->signals fire
if there is fire (localized)
* heat releases hydroge gas from the titanium hydride
* temperature felt locallly (ie local area)
* pressure increases to 40psi
* Closes the alarm switch ->signals fire
Fire warning controls
- illuminated FIRE master warning caption in Flashing Red
- warning bell
- Eng 1 or Eng 2 steady illuminated caption
- appropriate extinguisher operating handle illuminated
- Display **warning on EICAS or ECAM **
TEST BUTTON
- when pressed should indicate with a red light (means it’s working)
- if no alarm (test failed)
In continous loop detector
if a break between core and outer tube, then no alarm is given (test failed)
can still fly if 1 loop fails in flight. System switches to 1 loop operation (limited # of sector of flights)
In an gas filled detector
if integrity switch detector is open, no alarm is given (test failed)
Engine fire extinguisher (Power Plant)
2 bottle (2-shot) system on the wing (or cargo compartment)
Each bottle can send extinguisher to either engines
required by law
either engine is capable of receiving a total of 2 shots of extinguisher
filled with HALON (BCF or BTM)
TOILET:
* Also ther is a built in fire extinguisher in the toilet towel disposer.
- It is triggered by heat not smoke
what happens when you pull the fire handle for the affected engine?
by PILOT ACTION
Closes:
* the shut off valve in wing spar to isolate the fuel system
* the air bleed PRSOV
* hyrdraulic pump
* APU shut off valve
* closing of hte isolation and de-icing valves
Isolate
* hydraulic fluid
* engine drive electric generator
Arms
* extinguisher squibs ready for release
how it works
When the fire handle is rotated, the extinguisher bottle fires (electrically) and ruptures a retaining disc in the valve
This forces the piston to move in the squib to release the liquid agent and turns into gas
Each squib feeds a particular engine.
on each bottle (L and R)
Squib L for Engine 1
Squib R for Engine 2
Engine 1:
rotate anti clock wise
activate squib L
discharge contant into Eng 1.
Bottle L can no longer be used after discharge
Rotate fire handle clock wise
activate squib R
discharge contant into Eng 1.
Bottle R can no longer be used after discharge
If a bottle has been discharged into one engine, the bottole can no longer be used for the other engine
what are ‘photoelectric cells’
smoke detectors
detect the diffusion of a beam of light which occurs when the beam is interrupted by smoke.
the scattering of the light increases the ‘conductance’ of the cell and its outp is amplified to operate a warning circuit
